A Multiple Input Multiple Output (MIMO) system refers to a system for improving data transmission/reception efficiency using multiple transmission antennas and multiple reception antennas. MIMO technology includes a spatial diversity scheme and a spatial multiplexing scheme. The spatial diversity scheme is suitable for data transmission of a User Equipment (UE) which is in motion at high speed, because transmission reliability is increased or a cell radius is increased through diversity gain. The spatial multiplexing scheme can increase data transfer rates without increasing system bandwidth by simultaneously transmitting different data.
In a MIMO system, each transmission antenna has an independent data channel. The transmission antenna may mean a virtual antenna or a physical antenna. A receiver estimates a channel with respect to each transmission antenna and receives data transmitted from each transmission antenna. Channel estimation refers to a process of compensating for signal distortion due to fading so as to restore a received signal. Fading refers to a phenomenon in which signal intensity is rapidly changed due to multi-path and time delay in a wireless communication system environment. For channel estimation, a reference signal known to both a transmitter and a receiver is necessary. The reference signal may be abbreviated to RS or referred to as a pilot according to the standard applied.
A downlink reference signal is a pilot signal for coherent demodulation, such as a Physical Downlink Shared Channel (PDSCH), a Physical Control Format Indicator Channel (PCFICH), a Physical Hybrid Indicator Channel (PHICH), and a Physical Downlink Control Channel (PDCCH). The downlink reference signal includes a Common Reference Signal (CRS) shared among all UEs in a cell and a Dedicated Reference Signal (DRS) for a specific UE. The CRS may be referred to as a cell-specific reference signal. The DRS may be referred to as a UE-specific reference signal.
In arranging RSs on radio resources, there are several things to be considered, for example, the amount of radio resources to be allocated to the RSs, exclusive arrangement of DRSs and CRSS, the location of a control region in which a PDCCH etc. is arranged, and the density of the DRSs. If a large amount of resources is allocated to the RSs, the density of the RSs is increased and thus high channel estimation performance can be obtained. Meanwhile, if a small amount of resources is allocated to the RSs, although a high data transmission rate can be acquired, the density of the RSs is decreased and thus degradation of channel estimation performance may occur.
Accordingly, a method for efficiently arranging DRSs on radio resources so that channel estimation performance is increased and an increase of system overhead can be prevented in MIMO transmission is needed.